In my eighth grade there's this girl pregnant what should i do to talk to her with out sounding offensive ?

Talk to her like you would talk to anyone else. Just because she is young doesn't mean you can't ask about her pregnancy, just don't ask about the private details of it. Ask her how far along she is, if she can feel baby kicking yet, when she is due. She might find it refreshing that someone will talk to her about her pregnancy without wanting to into her business at the same time. You don't need to feel awkward around her just because she is a young mother. That being said, you shouldn't talk ONLY about her pregnancy, but just showing her that you are generally interested should be fine.